Steam Distribution
Contents
Pipe Expansion and Support
Any steam system must be fully supported, able to expand during operation and sufficiently flexible to allow movement as a result. This tutorial includes advice on different methods and full calculations.
Allowance for expansion
All pipes will be installed at ambient temperature. Pipes carrying hot fluids such as water or steam operate at higher temperatures.
It follows that they expand, especially in length, with an increase from ambient to working temperatures. This will create stress upon certain areas within the distribution system, such as pipe joints, which, in the extreme, could fracture. The amount of the expansion is readily calculated using Equation 10.4.1, or read from an appropriate chart such as Figure 10.4.1.
Example 10.4.1
A 30 m length of carbon steel pipe is to be used to transport steam at 4 bar g (152°C). If the pipe is installed at 10°C, determine the expansion using Equation 10.4.1.
Alternatively, the chart in Figure 10.4.1 can be used for finding the approximate expansion of a variety of steel pipe lengths - see Example 10.4.2 for explanation of use.
Example 10.4.2
Using Figure 10.4.1. Find the approximate expansion from 15°C, of 100 metres of carbon steel pipework used to distribute steam at 265°C.
Temperature difference is 265 - 15°C = 250°C.
Where the diagonal temperature difference line of 250°C cuts the horizontal pipe length line at 100 m, drop a vertical line down. For this example an approximate expansion of 330 mm is indicated.
Pipework flexibility
The pipework system must be sufficiently flexible to accommodate the movements of the components as they expand. In many cases the flexibility of the pipework system, due to the length of the pipe and number of bends and supports, means that no undue stresses are imposed. In other installations, however, it will be necessary to incorporate some means of achieving this required flexibility.
An example on a typical steam system is the discharge of condensate from a steam mains drain trap into the condensate return line that runs along the steam line (Figure 10.4.2). Here, the difference between the expansions of the two pipework systems must be taken into account. The steam main will be operating at a higher temperature than that of the condensate main, and the two connection points will move relative to each other during system warm-up.
The amount of movement to be taken up by the piping and any device incorporated in it can be reduced by ‘cold draw’. The total amount of expansion is first calculated for each section between fixed anchor points. The pipes are left short by half of this amount, and stretched cold by pulling up bolts at a flanged joint, so that at ambient temperature, the system is stressed in one direction.
When warmed through half of the total temperature rise, the piping is unstressed. At working temperature and having fully expanded, the piping is stressed in the opposite direction. The effect is that instead of being stressed from 0 F to +1 F units of force, the piping is stressed from -½ F to + ½ F units of force.
In practical terms, the pipework is assembled cold with a spacer piece, of length equal to half the expansion, between two flanges. When the pipework is fully installed and anchored at both ends, the spacer is removed and the joint pulled up tight (see Figure 10.4.3).
The remaining part of the expansion, if not accepted by the natural flexibility of the pipework will call for the use of an expansion fitting.
In practice, pipework expansion and support can be classified into three areas as shown in Figure 10.4.4.
The fixed or ‘anchor’ points ‘A’ provide a datum position from which expansion takes place.
The sliding support points ‘B’ allow free movement for expansion of the pipework, while keeping the pipeline in alignment.
The expansion device at point ‘C’ is to accommodate the expansion and contraction of the pipe.
Roller supports (Figure 10.4.5 and 10.4.6) are ideal methods for supporting pipes, at the same time allowing them to move in two directions. For steel pipework, the rollers should be manufactured from ferrous material. For copper pipework, they should be manufactured from non-ferrous material. It is good practice for pipework supported on rollers to be fitted with a pipe saddle bolted to a support bracket at not more than distances of 6 metres to keep the pipework in alignment during any expansion and contraction.
Where two pipes are to be supported one below the other, it is poor practice to carry the bottom pipe from the top pipe using a pipe clip. This will cause extra stress to be added to the top pipe whose thickness has been sized to take only the stress of its working pressure.
All pipe supports should be specifically designed to suit the outside diameter of the pipe concerned.
Expansion fittings
The expansion fitting (‘C’ Figure 10.4.4) is one method of accommodating expansion. These fittings are placed within a line, and are designed to accommodate the expansion, without the total length of the line changing. They are commonly called expansion bellows, due to the bellows construction of the expansion sleeve.
Other expansion fittings can be made from the pipework itself. This can be a cheaper way to solve the problem, but more space is needed to accommodate the pipe.
Full loop
This is simply one complete turn of the pipe and, on steam pipework, should preferably be fitted in a horizontal rather than a vertical position to prevent condensate accumulating on the upstream side.
The downstream side passes below the upstream side and great care must be taken that it is not fitted the wrong way round, as condensate can accumulate in the bottom. When full loops are to be fitted in a confined space, care must be taken to specify that wrong-handed loops are not supplied.
The full loop does not produce a force in opposition to the expanding pipework as in some other types, but with steam pressure inside the loop, there is a slight tendency to unwind, which puts an additional stress on the flanges.
This design is used rarely today due to the space taken up by the pipework, and proprietary expansion bellows are now more readily available. However large steam users such as power stations or establishments with large outside distribution systems still tend to use full loop type expansion devices, as space is usually available and the cost is relatively low.
Horseshoe or lyre loop
When space is available this type is sometimes used. It is best fitted horizontally so that the loop and the main are on the same plane. Pressure does not tend to blow the ends of the loop apart, but there is a very slight straightening out effect. This is due to the design but causes no misalignment of the flanges.
If any of these arrangements are fitted with the loop vertically above the pipe then a drain point must be provided on the upstream side as depicted in Figure 10.4.8.
Expansion Loops
The expansion loop can be fabricated from lengths of straight pipes and elbows welded at the joints (Figure 10.4.9). An indication of the expansion of pipe that can be accommodated by these assemblies is shown in Figure 10.4.10.
It can be seen from Figure 10.4.9 that the depth of the loop should be twice the width, and the width is determined from Figure 10.4.10, knowing the total amount of expansion expected from the pipes either side of the loop.
Sliding joint
These are sometimes used because they take up little room, but it is essential that the pipeline is rigidly anchored and guided in strict accordance with the manufacturers’ instructions; otherwise steam pressure acting on the cross sectional area of the sleeve part of the joint tends to blow the joint apart in opposition to the forces produced by the expanding pipework (see Figure 10.4.11). Misalignment will cause the sliding sleeve to bend, while regular maintenance of the gland packing may also be needed.
Expansion bellows
An expansion bellows, Figures 10.4.12, has the advantage that it requires no packing (as does the sliding joint type). But it does have the same disadvantages as the sliding joint in that pressure inside tends to extend the fitting, consequently, anchors and guides must be able to withstand this force.
Bellows may incorporate limit rods, which limit over-compression and over-extension of the element. These may have little function under normal operating conditions, as most simple bellows assemblies are able to withstand small lateral and angular movement. However, in the event of anchor failure, they behave as tie rods and contain the pressure thrust forces, preventing damage to the unit whilst reducing the possibility of further damage to piping, equipment and personnel (Figure 10.4.13 (b)).
Where larger forces are expected, some form of additional mechanical reinforcement should be built into the device, such as hinged stay bars (Figure 10.4.13 (c)).
There is invariably more than one way to accommodate the relative movement between two laterally displaced pipes depending upon the relative positions of bellows anchors and guides. In terms of preference, axial displacement is better than angular, which in turn, is better than lateral. Angular and lateral movement should be avoided wherever possible.
Figure 10.4.13 (a), (b), and (c) give a rough indication of the effects of these movements, but, under all circumstances, it is highly recommended that expert advice is sought from the bellows’ manufacturer regarding any installation of expansion bellows.
Pipe Support Spacing
The frequency of pipe supports will vary according to the bore of the pipe; the actual pipe material (i.e. steel or copper); and whether the pipe is horizontal or vertical.
Some practical points worthy of consideration are as follows:
- Pipe supports should be provided at intervals not greater than shown in Table 10.4.3, and run along those parts of buildings and structures where appropriate supports may be mounted.
- Where two or more pipes are supported on a common bracket, the spacing between the supports should be that for the smallest pipe.
- When an appreciable movement will occur, i.e. where straight pipes are greater than 15 metres in length, the supports should be of the roller type as outlined previously.
- Vertical pipes should be adequately supported at the base, to withstand the total weight of the vertical pipe and the fluid within it. Branches from vertical pipes must not be used as a means of support for the pipe, because this will place undue strain upon the tee joint.
- All pipe supports should be specifically designed to suit the outside diameter of the pipe concerned. The use of oversized pipe brackets is not good practice.
Table 10.4.3 can be used as a guide when calculating the distance between pipe supports for steel and copper pipework.
The subject of pipe supports is covered comprehensively in the European standard EN 13480, Part 3.